There are several explanations for your bettas color. The only bettas that I know change color are those with the marble and red loss gene. Unless you consider red wash a change in color. More than likely, yours is a turquoise betta that sometimes looks blue, and other times green because of the way light reflects off its irridescence. The horizontal stripes mean that the betta is stressed out or frightened. Often times they are duller in color in the mornings or late at night. Temperature and foods also effect a bettas color intensity. Warmer temps and better foods brighten a betta up quite a bit.
Alexa